X-ray fluorescence (XRF) is a technique used to measure the elemental composition of a sample. The sample is excited by a source of x-rays, and emits its own characteristic x-rays. A detector is responsive to the x-rays emitted from the sample. An analyzer processes the output signals produced by the detector and divides the energy levels of the detected x-ray photons into energy subranges by counts of the number of x-ray photons detected to produce a graph depicting the x-ray spectrum of the sample.
In XRF, the sample is irradiated with x-rays either from a radioactive isotope source or, more commonly, from an x-ray tube. A typical isotope source, for example Fe-55, has a limited range of emitted x-ray energies which cannot be changed. Therefore to excite a wide range of elements efficiently, the use of multiple sources is required. See U.S. Pat. No. 6,859,517 incorporated herein by this reference. In systems where an x-ray tube is used to provide the exciting radiation, it is often sufficient to use only a single x-ray tube because the x-ray energy distribution can be changed by controlling the high voltage supply and by applying filters between the source and sample. The output of an x-ray tube is composed of discrete lines, which are specific to the anode material of the tube, superimposed onto a continuum background of energies which extend up to the maximum energy of the supplied high voltage. By changing the anode material of the tube, it is possible to select characteristic lines at different energies thus avoiding potential overlaps between elemental lines from the anode and those in the sample, or to choose a line which can efficiently excite a particular element in the sample.
A general purpose XRF system typically employs a single x-ray tube equipped with high voltage control and uses a range of different filters to provide an instrument which can be applied to a whole range of different analytical problems. It is possible to enhance the performance of an instrument for a specific application by changing the tube anode material to produce characteristic lines which more efficiently excite the element(s) of interest. When the range of elements to be analyzed is limited, it is also possible to create a monochromatic x-ray beam which can give exceptional sensitivity for the chosen elements. An example is the analysis of low concentrations of sulfur (<1 ppm) in diesel fuel. Because diesel fuel is a well-known hydrocarbon mixture with a well defined chemistry, may be no need to measure any other elements besides sulfur in these samples. The effects of the hydrocarbon matrix are included in the calibration method. The matrix does not change appreciably for different samples and there are no other elements of appreciable concentration (besides the base hydrocarbon elements C, H, O) thus there may be no need to measure other elements besides sulfur.
A general problem in XRF analysis is the need to analyze a wide range of elements in a sample whereby high concentrations of some elements mask the presence of, or interfere with, low concentrations of other elements that also must be measured. A high concentration of an element can produce a large response in the measured energy spectrum. The large peak in the spectrum generates excessive background in the detector that reduces the signal-to-background ratio of other elements. The ideal case is to measure the sample with one source configuration where the higher concentration elements are measured, and then measure the sample with a different source where the high concentration elements are not excited by that source thus minimizing the background in the spectral region of the other elements of interest. There are numerous examples of this general problem. For example, in environmental soil samples it is desirable to measure low concentrations (<50 ppm) of Cr in soil. Often, however, iron concentrations in soil exceed several percent. In an XRF spectrum, the iron peak is centered at 6.4 keV and the Cr peak is centered at 5.4 keV. The state of the art in small semiconductor detectors typically used in commercial devices have a relatively high iron peak that produces a background “tail” into the chromium region. This background tail obscures the low concentrations of chromium that need to be measured.
Recently, there is also a need to analyze lubricant and fuel oil samples for the presence of S, Cr, V, Fe, Ni, Cu, Zn and other elements. These elements are either naturally occurring in the fuels and oils or are present as additives or contaminants (wear metals). There is also a need to analyze fuel samples for the presence of catalysts fines (catfines), typically, silicon, and aluminum. In an XRF system with a single x-ray tube, the spectrum would be dominated by the high levels (0.5-5.0%) of sulfur in the sample which makes the analysis of low levels (<80 ppm) of aluminum and silicon extremely difficult if not impossible. In spectrum from a standard XRF measurement, a high sulfur peak is seen in the region around the x-axis value of 2,300 eV. The area of interest for aluminum and silicon is approximately 1,500 and 1,750 eV respectively. The background “tailing” from the high sulfur concentration extends down to past the lower region of the spectrum where Al and Si are to be measured. The only way to achieve the required sensitivity for Al and Si (<80 ppm detection limit) is to produce an x-ray spectrum that eliminates the high sulfur peak.
One practical method of analyzing the small amounts of aluminum and silicon in samples containing high levels of sulfur is to use a low energy source of x-rays which does not excite the sulfur atoms. A simple approach would be to operate the x-ray tube at a voltage below the absorption edge of sulfur 2.472 keV but this approach would yield an extremely small number of useful x-rays from the source. A more efficient solution is to operate a molybdenum anode tube at a typical operating voltage e.g. 25 keV, and then monochromate the output beam to include only the Mo-L lines at 2.29 keV. The use of a monochromatic source to measure the catfines is discussed in co-pending U.S. application Ser. No. 11/585,367. The more general use of a monochromator to produce a mono-energetic beam of x-rays on a sample via a curved crystal is presented in U.S. Pat. No. 4,599,741 (Wittry et al.). Wittry et al describe a curved crystal structure combined with an x-ray source to produce a mono-energetic beam that would be effective for various XRF analysis applications. This patent does not specifically discuss the need to measure multiple elements sequentially, with multiple sources. Nor does it address the specific problem solved by our invention namely the ability to measure low concentrations of one group of elements in the presence of high concentrations of other elements that may interfering with the first measurement. For an instrument to be capable of measuring all the elements required in fuel oils and lubricants, it needs to combine the capabilities of standard tube excitation and also monochromatic excitation. In some configurations and applications, this could possibly be achieved by using a single x-ray tube to excite the sample, directly or via a monochromatic pathway, and could employ some mechanical devices to switch between one mode and another. However, for the situation where one of the elements to be measured by a direct beam is sulfur, and the elements to be measured by the monochromatic beam are aluminum and silicon, it is not possible to use the same tube anode material. This is because the Mo-L line (2.29 keV) lies at almost the same energy as S (2.307 keV), which means that it would not be possible to quantify the amount of sulfur in a sample. Finally, an accurate determination of the amount of aluminum and silicon in a sample depends on knowing the amount of sulfur and other elements in a sample. Therefore, it is important to quantify all elements in a material using the same instrument, so that results from multiple measurements can be combined into a final result.
A scientist using a laboratory based XRF system may be able to adjust their determinations when analyzing a sample based on the scientist's advanced knowledge of chemistry, physics, and the specifications of the XRF system used. But, a fuel sample would typically be analyzed in the field often by less knowledgeable technicians.